The invention relates to a method of characterizing the rheological properties of a plastic material, and more particularly to a rheometer intended for the determination of the working-up capacity of a molten visco-elastic polymeric material.
It is known that information on the shear modulus and on the viscosity of a polymeric material is of extreme importance in characterizing commercial plastics, particularly thermo-plastics, since these properties determine the specific applications for which the plastics are intended. Thus, thermo-plastic resins intended for extrusion have visco-elastic properties which are very different from those intended for injection-moulding.
These rheological phenomena have already given rise to the study and development of apparatuses capable of providing data, the interpretation of which makes it possible to calculate the desired properties. Thus U.S. Pat. No. 3,559,464 describes a rheometer for continuous monitoring, arranged in parallel with the main polymer flow line and comprising (a) a heat-controlled block provided with a first capillary tube and with a second capillary tube of the same diameter but of a greater length than the first tube, (b) a pump of variable speed connected to the block and inserted between the two capillaries, and (c) differential pressure gauges fitted on the capillaries. This device constitutes a pressure regulation system which is intricate and difficult to maintain, and suffers the disadvantage of allowing measurement of the rheological properties for only a single shear-level of the visco-elastic material. From this standpoint it therefore provides incomplete results and allows only an approximate determination of the working-up capacity of the polymers.
A first object of the inventon is therefore to provide an apparatus which can, by measurements at different shear-levels, determine with great precision the working-up capacity of a molten visco-elastic polymeric material. A second object of the invention is, when a lesser precision in the determination of the rheological behavior of a polymer is acceptable, to provide an apparatus whose construction and maintenance are easier than those of known apparatuses and which can more easily be adapted to the direct and speedy monitoring of very large polymerization units.